I am sure there are others with more experience regarding the L series sealing characteristics, but I know of people who have had issues with the sand and salt water spray negatively affecting the EF-S (non-weather sealed) lenses. If you are going to be at a beach, it might be a consideration.

Recently I was able to take some beach shots, and when I got home there was a film of salt spray on my lenses, one was the 24-105 and the other the 70-200. Both cleaned up very easily with a lens wipe, but not sure what the 17-55 would do in the same situation.

My 17-55 just arrived Friday, so haven't had much time to compare it, but so far it appears nearly identical IQ to my 24-105 and 17-40. My impression so far is that the 17-55 is a lot more fragile than either of the other two L lenses, but takes great pictures!

I am using a crop sensor camera (60D).

Hope this helps!
Brian